In recent years, with rapid penetration of cable broadband service such as ADSL, FTTH and the like in ordinary homes, the so-called rich content service has become widespread using speech, video and music beyond textual information, and an information amount for a person to handle has increased. Also in mobile communication, rates of using non-speech conversation service have been sharply increasing such as mobile Web, music distribution and the like, and demands for broadband wireless communication have grown as in cable communication.
For such requests for broadband mobile communication, various studies have been made. Among the studies, OFDMA (Orthogonal Frequency Division Multiple Access) is an access scheme receiving attention in terms of spectral efficiency, and fading resistance. OFDMA is the scheme for by using characteristics of OFDM for arranging densely a large number of orthogonal subcarriers at intervals of a reciprocal of a signal duration, allocating an arbitrary number of subcarriers (or a frequency channel comprised of a group of successive subcarriers) with good characteristics to each terminal corresponding to reception characteristics varying with terminals in multipath environments, and thereby further increasing substantial spectral efficiency.
The OFDM modulation scheme is adopted in wireless LAN specifications such as 5 GHz-band IEEE802.11a and the like, where channels having an occupied bandwidth of 16 MHz or more are arranged at intervals of 20 MHz. Accordingly, a region of 3 MHz or more without carriers exists between channels. Further, basically, a single terminal uses a single channel, a band for each terminal to be able to modulate and demodulate is the same as a band to communicate and is always constant, and therefore, this scheme does not correspond to OFDMA.
For OFDMA, any system put into practical use has not existed at the present time, but OFDMA is the system for allocating subcarriers and frequency channel of optimal reception states to each terminal from a wide band, and therefore, any proposals agree with one another in the concept that a band for a terminal to use in communication varies with a band for the terminal to be able to modulate and demodulate being the maximum band. Accordingly, a required frequency band is determined from the maximum transmission rate requested in a system, and communication apparatuses in the system are required to be able to collectively modulate and demodulate the frequency band. For example, in “IEICE Technical Report RCS2004-85(2004-06)”, “2004 IEICE General Conference B-5-64” and the like, a frequency band of 100 MHz is expected to be required per user to realize 100 Mbps, and it is proposed to arrange subcarriers evenly in the band.
Non-patent Document 1: IEICE Technical Report RCS2004-85(2004-06)
Non-patent Document 2: 2004 IEICE General Conference B-5-64